1. Field of the Invention
The present invention relates to a subtalar implant, and in particular, an implant that can be used to correct a valgus deformity of the foot.
2. Background Art
For many years, surgeons have been attempting to correct valgus deformities of the foot—e.g., pes planus, or flatfoot—using a number of different techniques. One technique is to use a subtalar implant that is inserted into the tarsal sinus to reposition the calcaneus relative to the talus. One such device is described in U.S. Pat. No. 6,168,631 issued to Maxwell, et al. on Jan. 2, 2001. The implant described in Maxwell et al. is a metallic screw having external threads with slots formed in the threads. One limitation of the implant described in Maxwell, et al., is that there is no smooth surface on which the ankle bones can articulate. In fact, the talus and the calcaneus articulate on the same sharp-edged threads that are used to secure the implant within the joint. Such a configuration may lead to irritation of the articular bone and surrounding tissue.
One attempt to deal with this problem is described in U.S. Pat. No. 5,360,450 issued to Giannini on Nov. 1, 1994. Giannini describes an implant configured for insertion into the tarsal sinus for correction of pes planus. The Giannini implant is a two-piece device consisting of a cylindrical body and a screw which is configured for insertion into the cylindrical body. The cylindrical body includes a longitudinal incision which allows the body to expand when the screw is inserted into an axial hole. The entire implant is made from a bioresorbable material, such that removal of the implant is not necessary, rather, it is designed to be resorbed into the patient's body.
Although the Giannini implant does not require the ankle bones to articulate on metal threads, it nonetheless has a number of limitations. For example, the outer surface of the cylindrical body includes a plurality of grooves which are intended to provide a location for the growth of fibrous tissue. Necessarily, a plurality of rings abut the grooves such that the bones do not have a smooth surface on which to articulate. In addition, the implant described in Giannini relies on a wedge-effect using a bioresorbable material to secure the implant. Thus, the Giannini implant does not have the benefit of metal threads to securely hold the implant in the joint space.
One implant which uses a combination of metallic and polymeric components is described in U.S. Pat. No. 6,136,032 issued to Viladot Perice et al. on Oct. 24, 2000. The implant described in Viladot Perice et al. is a three-piece implant that is configured for insertion into the tarsal sinus. The Viladot Perice et al. implant includes a metal cone which is drawn up toward an implant head, thereby expanding an outer polyethylene cylinder. The polyethylene cylinder includes a plurality of fins which are shaped as barbs, tapering away from an outer surface of the cylinder and returning abruptly to the cylinder, thereby creating a sharp edge. The implant described in Viladot Perice et al. relies on a wedge-effect and the polyethylene fins to secure the implant. Thus, Viladot Perice et al. implant does not have the advantage of metal threads to secure the implant within the joint space. Moreover, the Viladot Perice et al. implant includes a plurality of sharp-edged fins which may irritate the joint tissue as the bones articulate.
Therefore, a need exists for an implant which can be used within the tarsal sinus that provides the advantage of threads to secure the implant in the joint space, and at the same time, provides a smooth surface on which the bones may articulate, thereby inhibiting friction and irritation within the joint.